Loud-speaker structure



July 8, 1952 L. s. DoulerrI LoUwsPEAKER STRUCTURE 2 SHEETS SHEET l *IFiled NOV. 18, 1947 III? III?

INVENTOR.

ATTORNEY Eo/v STEWART oouar m@ L m@ nl@ Patented July 8, 1952 UNITEDSTATES PATENT OFFICE LOUD-SPEAKER STRUCTURE Leon Stewart Doubt,Altadena, Calif.

Application November 18, 1947, Serial No. 786,563

4 Claims. 1

This invention is concerned with loud speakers and provides improvementsin loud speaker arrangement to the end that improved low frequencyresponse is obtained. More particularly, the invention provides a novelarrangement of loud speakers in a cluster to the end that the responseof the cluster exceeds substantially the response of any individual loudspeaker in the cluster.

The advantages of the invention are manifold, i'

including not only improved performance but also lower unit cost,lighter weight with unbraced cabinetry and non-reflex construction andsmaller dimensions.

In its simplest form a loud speaker includes a cone of ilexible materialsuch as paper, fastened at its edges rigidly to a baffle or the like andhaving a magnetizable diaphragm at its apex. A magnet is disposed behindthe diaphragm and this is energized by a coil connected to the output ofan audio-amplier or the like and either transformer or resistor coupled.The movement of the diaphragm due to iiuctuations of the field of themagnet vibrates the cone and thus produces sound. Dynamic speakerslikewise employ a cone, but employ a voice coil instead of thediaphragm. The voice coil is wound on a small insulating tube thatslides back and forth along the magnet and is energized by theamplifier. The magnet, in the case of a dynamic speaker, may be either apermanent magnet or an electromagnet that is steadily energized. Thecone is attached to the tube by a spider of springy material. Thefluctuating magnetic iieldv set up in the voice coil reacts with thesteady eld of the magnet and moves the voice coil up and down themagnet. The spider permits the voice coil to move and forces it to comeback once the pull ceases. The cone connected to the voice coil movesWith it and vibrates to produce audible sound. The dynamic speaker issuperior to the simpler type in that rattle due to the diaphragmstriking the magnet is eliminated, but both types depend upon movementof the ilexible cone to vibrate and produce sound Waves.

The effectiveness of a vibrating device such as a speaker cone toproduce an audible tone is determined by its ability to set in motion amass of air. In the low frequency range of the audible frequencyspectrum, the mass of air to be moved is large-so large in fact thatmany prior loud speakers and loud speaker systems are unable to respondfundamentally to tones in the low frequency range, i. e. below 100 C. P.S. One Way of increasing the mass of air is to increase the size of 2the cone in an individual speaker. However, this approach has severaldisadvantages:

l. Paper, of which loud speaker cones generally are fabricated, is notsuliciently strong to permit cone diameters much in excess of 18 inches.

2. As the size of the cone is increased, the ability of the speaker torespond to tones of a higher frequency, i. e. those of 4000 to 5000 C.P. S. and up, is severely reduced.

3. Large speakers and their attendant magnetic or field coils areunwieldy and the baiiles upon which such oversize speakers are mountedare too large for many uses.

.As a result of my investigations, I have developed a loud speakersystem in which a plurality or cluster of speakers is mounted on asingle bale in such fashion that the acoustic result in terms oi basefrequency is closely analogous to that obtainable from a single largespeaker having a cone area equal to the areas of the cones of thecluster. This result is achieved, however, without sacrifice of highfrequency response. Thus the low frequency response of the clusterapproaches the theoretical maximum while the high frequency response isnot attenuated, as would be the case with a single speaker of equivalentcone area. In fact, because of variations in the mechanical resonancefrequency of the individual speakers mounted in clusters in accordancewith my invention, the high frequency response of the cluster generallyexceeds in accuracy the response of any single component speaker in thecluster. Moreover, the low frequency response of the cluster is muchlower than that of any individual speaker of the cluster.

In accordance with my invention the individual cones of a plurality ofspeakers are arranged in a cluster on a common baffle, with the edges ofthe cones very close together. This mounting of the speaker cones closetogether, i. e. with the distance between cone rims less than the coneradius and preferably less than 3 inches, is hereinafter referred to astangential mounting.

The ability of the cone cluster of the invention to reproducefundamentally lower tones than any which can be produced by any of theindividual speakers of the cluster is apparently due to an acousticeffect rather than to an electronic or magnetic effect. I havedetermined experimentally that the acoustic effect is at a maximum whenthe speakers are mounted so that the rims of the speaker frames areactually in contact, i. e. tangential to each other; that the eifect isdecreased sharply when the distance between the acoaseo 3 speakers isincreased; and that the effect is completely absent as soon as thedistance between the rims of the speakers exceeds the radius of thespeakers involved.

In short, my invention contemplates a combination in a loud speakerstructure which comprises a bale, at least two speakers mounted rigidlyin the baiile with their edges close together, the distance between theedges of the speakers being less than the radius of any of the speakers,with common means for energizing the speakers. As indicated above, it ispreferable that the speakers be mounted tangential with each other.

Some improvement in low frequency response is obtained with only twospeakers mounted tangentially. If two speakers which will respondindividually to no frequency lower than 200 cycles are mountedvtangentially in a common bafe and connected to a common energy source,the two together will respond fundamentally to 100 cycles. If a thirdspeaker is added to the cluster of two no additional improvementresults'. However, if four such speakers are combined, preferably in asymmetrical two-row group, these will respond to 50 cycles. Eight suchspeakers, mounted tangentially and symmetrically in a common baffle willrespond to cycles.

The addition of one, two or three speakers to a cluster of foursymmetrical and tangentially mounted speakers does not bring anyimprovement in low frequency response. However, when a fourth speaker isadded to give a total of eight the response is improved by one octave.In sum, each time the number of speakers is doubled, the lowestfrequency to which the cluster will respond fundamentally is lowered byone octave.

While the foregoing relationship actually exists, there is a practicallimit beyond which a gain in fundamental frequency response isimpracticable regardless of the number of speakers. This limit seems tobe a function of the individual speakers. However, a grouping of eightor nine speakers is sufficient for all practical purposes, because evenemploying cheap individual speaker cones which do not have a frequencyresponse below 200 C. P. S., the combination of eight such speakers inaccordance with the invention permits a lowering of frequency responseto 1/8 of 200 or 25 C. P. S. No musical instrument, except the singlepipe of the largest organ, is capable of producing tones below afrequency of 25v C. P. S. andno human voice is capable of producingtones below this frequency. So, from a practical standpoint thereis noneed for increasing the number of speakers above eight or nine.

e The preferable number of speakers for the practice of vmy invention isnine. As indicated above, optimum acoustic result is obtained with eightspeakers and no further improvement in frequency response is obtained byadding the ninth. There are advantages however, since nine speakers-permit symmetry to be more easily obtained and impedance matching moreeasily accomplished. Thus I prefer to employ nine identical speakersarranged in three rows of three each. These speakers are wired halfseries half parallel, so that the voice coil impedance of the totalcluster is the same as that of the speakers taken singly. With s uch anarrangement, standard amplifiers designed to match such single voicecoil impedance may be used to energize the 4 cluster and no specialtransformer coupling or' other device is required between the amplifierand the cluster.

In addition to the foregoing considerations, the following observationsshould be made:

l. If the speakers are mounted tangentially, but not symmetrically withrespect to two axes perpendicular to each other in the plane of thebaffle, the improvement in lowering of frequency response tends to bediminished and in some cases lost.

2. The type of speakers which compose the cluster is not critical.Either permanent magnetic or field type speakers may be employed.Improvement of base response occurs regardless of the diameter of theindividual speakers, but for matching purposes it is preferable that allof the speakers of the cluster be substantially identical.

3. Since the low frequency response can be brought to almost any desiredlevel in accordance with the invention it is possible to employ sqeakershaving hard or even metallic cones. Such speakers generally have abetter middle and high frequency range response than is the case withsofter paper cones. The base response resulting from the clustering ofthe speakers in accordance with the invention may thus be employed tocompensate for the usual lack of low frequency response in this type ofspeaker while retaining or even improving response at middle and highfrequency.

4. The only baling requirements are that the cluster of speakers bemounted on a single bafe of a dimension large enough to extend past theoutside edge of the cluster. In other words, the space between thespeakers of the cluster (which should be as small as possible) should bebaifled. No noticeable effect is obtained by further baffling and thereis no gain in true response by reflexing the speaker cluster.

5. Since a large mass of air is set in motion by the tangential speakercluster of the invention, no restriction of air movement should beimposed by the cabinet. This is true of the back of the unit as well asof the front. Thus a solid backed cabinet with no space for expansionand contraction of the body of air contained within the cabinet willhave a damping effect, especially with respect to the low frequencyresponse of the cluster.

6. The amplifier which energizes the cluster of speakers of theinvention cooperates with it and the maximum theoretical range offrequency response of the clustered speakers is not attained in practiceunless the amplifier is capable of delivering an equivalent range to thespeakers. The amplifier, therefore, should have a at frequency responseover the frequency range of the speaker cluster. It should also have afiat power output over this range. For many good commercial ampliers,frequency response is said to be iiat or linear from 20 or 30 to 10,000cycles and such afrequency response actually is available, in terms offrequency. However, if available power in the amplier is plotted againstfrequency there will usually be sharp deviations from linearity. Anaverage commercial 15 watt output amplier may produce l5 watts at somefrequency or even over a certain frequency range, say 400 to 2000 C. P.S., but at frequencies above ,and below this range the power output willdecrease until the outputof 20. cyclesnmay be aslow aspone-half watt andat 10,000 cycles, one or two watts. The

full response of the speaker structure of the invention is not availableif such an amplifier 1s employed, for linearity of power output as wellVas frequency is required for optimum results. Accordingly, I prefer toemploy an amplifier which has a nat power output or response over theaudible range. In short, the available power should be linear over theaudible range, say from to 15,000 cycles.

7. Because of the extremely low frequency response obtainable with theloud speaker cluster of the invention, faults of amplifier design whichwould not be apparent with conventional loud speakers become noticeable.If a conventional amplifier is employed with the clustered speakers ofthe invention there is a tendency for increased hum. This hum orunfiltered noise from the average conventional ainplier is notobjectionable with conventional speakers, but with the speaker structureof the invention what was previously considered as a quiet amplifier isfrequently shown to have considerable low frequency noise. Therefore, inorder to obtain optimum results the amplier employed with the speakerstructure of the invention should have complete filtering of lowfrequency noise and otherwise be as free from noise as possible.

8. Coupling of the loud speaker structure of the invention to theamplifier circuit is important. Transformer coupling is preferable andthe construction of the transformer is important since it must meet therequirements of linearity in terms of both frequency and power output.

These and other aspects of the invention will be more thoroughlyunderstood in the light of the Fig. 3 is a diagram illustratingtangential.

mounting of two speakers;

Fig. 4 is a diagram showing tangential and symmetrical mounting of fourspeakers in a preferred arrangement;

Fig. 5 is a diagram showing tangential and sym- -metrical mounting ofeight speakers in accordance with the invention; and

Fig. 6 is a wiring diagram showing a preferred form of amplifierconnected to the input of the nine speakers of the cluster of Figs. 1and 2.

As shown by Figs. l and 2, nine conical speakers I0, II, I2, I3, lli,I5, It, Il, d, are arranged symmetrically in three rows of three in asingle baille I9. This baiile may be at throughout or, as shown in Fig.2, may be composed of rigidly joined tilted segments for increasing thebroadcast angle, which is sometimes desirable, for example, in anauditorium. As shown in Fig. 1, the speakers are disposed with theircone edges or rims close together and substantially tangential to eachother.

The half series, half parallel electrical connection of the voice coils(which individually have impedances of 6-3 ohms) of the speakers is seenin Fig. 6. There are three series of three speaker each, the threeseries being connected in parallel with each other. In such anarrangement, the impedance of the total cluster of nine speakers is thesame as that of any of the individual speakers, assuming them all to beequal.

A variety of amplifier circuits which meet the requirements of thespeaker cluster may be employed. Thus, the amplifier should haveresponse which is substantially linear over the audible range both interms of frequency and in terms of power. By way of example, if theamplifier has a rating of 15 watts it should be capable of producing 15watts over the audible range of say, from 20 or 30 to 15,000 cycles.Otherwise the available frequency response of the clustered speaker unitis not employed to full advantage.

The amplier of Fig. 6 fulfills the foregoing requirements. It consistsof three audio-amplification stages 30, 3l, 32 and a power supply 33.

The first stage is resistance-coupled to the signal input through acondenser CII] and a potentiometer RI the control grid of a 6SJ7 pentodeof the first stage being connected to the slider of the potentiometerfor volume control purposes.

The first and second stages are resistancecoupled through a condenser C3and a resistance R5, the tube of the second stage being a 6J 5 triode.

The third stage contains two 6L6G tetrodes connected in push-pull inorder to obtain requisite power output, and the second stage isresistance-coupled to it through a condenser C4 and resistances R9, Rl0, with the cathodes of the two tubes connected to the grounded midpointbetween these resistors through the resistancecondenser combination RII, CS. The output of the third stage is transformer-coupled to the Voicecoils of the speakers through a transformer TI, the primary of which iscenter-tapped because of the push-pull arrangement of the third stage.

The power supply consists of a transformer T2, the primary of which isenergized by line current through a fuse and switch. The transformer hasa low voltage center-tapped secondary winding which supplies heatingcurrent at about 6.3 volts to the filaments (not shown) of all tubes.The transformer also has a center-tapped high voltage secondary windingconnected to a conventional full-wave rectifier tube VI, having afilament energized at 5 volts by a third secondary winding. The outputof the rectifier is the high voltage supply for the amplifier at about350 volts and is sent to the three stages through a resistance RMprovided with grounded condensers C8, C9 at its ends.

To consider the first stage in greater detail, .the cathode andsuppressor grid of its tube are connected for feed back purposes to theoutput side of the transformer TI through a resistance RI3 in serieswith the condenser CI. 'I'he cathode is grounded through a potentiometerR2, the slider of which is connected between the resistance R13 and thecondenser CI. Bias for'the screen grid of the tube is obtained byconnecting this grid to the power supply through the resistor R3, with acondenser C2 connected to ground between resistor and screen grid. Theplate is energized through a plate resistor R4.

The cathode of the tube of the second stage is coupled to the thirdstage'through a resistance RB and a condenser C5 and connected to groundthrough another resistance RI. The plate of this stage is connected tothe power supply through a plate resistor R8, and an interstage resistorRI2, through which power is also supplied to the rst stage. A condenserC1 is connected to ground between resistors R8 and RIZ.

In order to form a clear idea of the values of .condensers, resistors,etc., employed in the circuit, attention should be directed to thefollowing table:

Table Condensers Resistors 01;'25 mid. 25 v. (J2-.2 mrd. 400 v.

Rl 500 K pot.

R2 1000 ohm pot. WW.

(D3-.(7)5 mdf. 000 v. R3 1 mcg. t w. (14e-.1 mfd. 600 V. R4 270 K t W.(l5-.1 mid. 600 v. R5 .5 meg. t w. C6--50 mid. 25-50 V. R6 2400 ohm t'W. C7--16 mid. 450 v. R7 47 K t w. @8G40 mid. 450 V. R8 47 K 5c W.Cil-40 mid. 450 v. R9 270 K l w. C10-.05 mid. 600 V. R10 270 K l W.

R11 250,0hm 10 W R12 l5 K 1 W. R13 15 K la W.

The amplifier just described has a gain of 42 db with an output of notless than 15 watts within 1 db through a frequency range of 20 to 12,000cycles. There is less than one percent (1%) total harmonic distortion.The frequency response of this amplifier is within 1/4 db from 18 to22,000 cycles and within 2 db from 15 to 52,000 cycles. The noise level,as determined experimentally, is more than 60 db below one volt,measured with 10 ohms of pure resistive load across the output of theamplifier.

At some frequency, determined by the mechanical construction of thespeaker and the character of the amplifier which energizes it a singlespeaker of the prior art tends to blow, i. e. to resonate. Thus manyspeakers tend to fblow at about 80 to 110 or 120 C. P. `S. I havediscovered that even employing a speaker which tends to peak, i. e.increases its output at some frequency in the audible range, the speakerstructure of the invention is desirable, Vbecause the several speakersof the combination tend to damp each other mechanically, even though nodamping action is provided by the amplifier.

Nevertheless, when the speaker structure of the invention is to be 'runat something approachingk maximum output in terms of power, it isdesirable to employ an amplifier which has as a substantially flatfrequency and power response throughout the audible range, and thisresult can -bestbe yobtained by increasing nega-tive voltage feedback toa point considerably above the customary level, say to 15 db or more.Ar-1increase above 15 db, say to 601db is quite 'within the -frange oflpractice of the invention. However `it -must be recognized that suchamounts of feed- 'back are only obtained through sacrice of gain vandrequire care in engineering 'of the amplifier -cir'cuit to .preserveperformance and avoid stability, i. e. tendency to oscillate.

The high degree of `negative voltage feedback which is involved in thepreferred practice of the invention requires an output transformer ofexcellent characteristics. Thus the transformer Tl of Fig. 6 at the 15watt level has a fiat frequency response from .-10 to 42,000 C. P. S.within 1 db with power curve flat from 20 to 15,000 CfP. S. within 1 db,the percentage of harmonic distortion at the 15 Watt levelbe'inglessthan 1%. With such a transformer, the 0feedback in the circuit of Fig.`6 may be from l5 to 30'db, and preferably is about db.

As indicated heretofore, some advantage accrues to the practice of -theinvention with 'bnly two speaker 'cones mounted "on the 'baie 'intangential arrangement. For, as illustrated in Fig. "3, if `the -basefrequency Aof 'a `single speaker 8 is C. P. S. the mounting of two suchspeakers 40, 4l side by side on a single baffle 42 in tangentialarrangement will reduce the base frequency response to 50 C. P. S.

Really outstanding results in the practice of the invention begin whenfour speakers 43, 44, 45, 46, are arranged tangentially andsymmetrically in a baiiie -41 as shown in Fig. 4. Thus with fourspeakers having a base frequency response individually of 100 C. P. S.,the base frequency of the cluster will be reduced to 25 C. P. S.

Further advantage accrues with still larger speaker clusters, one sucharrangement with eight speakers, 50, 5l, 52, 53, 54, 55, 56, 51, beingshown in Fig. 5. Here again it will be observed that the speakers aredisposed tangentially on a common baffle 58 and arranged symmetricallyabout both a vertical and a horizontal axis. The base frequency responseof these eight speakers is the same as that obtainable with ninespeakers of the same type. However, impedance matching is somewhat moredinicult with the arrangement of Fig. 4 and so for practical purposes itis desirable to add an additional speaker to make a total of nine and toarrange these in three rows of three, as in Fig. 1.

The speaker cones need not be of the same diameter, but this arrangementis preferable. Moreover, it is desirable for purposes of impedancematching and ease of assembly and design, to employ speakers that arenot only equivalent mechanically but also equivalent electrically.

The invention makes it possible to obtain frequency response over theentire audible range with speakers of small size which heretofore havegiven relatively poor reproduction. Thus, by combining lsay nine cheapspeakers, in accordance with the practice of the invention, results maybe obtained which are superior to those which are obtainable with themost expensive single speakers heretofore available. Naturally enough,the better the individual speaker the better the total result, sincesome speakers are individually better than others in terms of linearityof response throughout the audio spectrum.

l claim:

1. In a loud speaker structure of non-renex construction, thecombination which comprises a single stationary baffle, ninesubstantially similar cone speakers mounted at their rims rigidly andsymmetrically to vthe bale in three rows of three each and substantiallytangential to each other, and common vmeans for energizing the speakerssimultaneously and substantially equally, the individual 'speakers ofthe rows being connected electrically in series with the rowselectrically connected in parallel with the energizing means.

l2, In a loud speaker structure of non-reflex construction, thecombination which comprises a single stationary bale, a cluster of'speakers arranged in parallel rows, with the same number of speakers ineach row as there are rows, the speakers being rigidly fastened to thesingle baffle at their peripheries Withl their center portions movable,and with their peripheries substantially tangential to each other, andcommon means for energizing the speakers simultaneously 'andsubstantially equally, the speakers of each Vruw being electricallyconnected in Vparallel with the Venergizing means.

3. Apparatus according to claim 2 inwhich the energizing Vmeans is anamplifier having fa substantially flat power response over the frequency'response range of the cluster.

9 4. Apparatus according to claim 2 in which the energizing means is anamplier having a substantially flat power response over the frequencyresponse range of the cluster and having a nega tive voltage feedback ofat least 15 db.

LEON S'IEWART DOUBT.

REFERENCES CITED The following references are of record in the Iile ofthis patent:

UNITED STATES PATENTS Name Date Williams June 24, 1930 Number NumberNumber

